Can I attend classes at TU or FU if I am not yet enrolled at these universites?

Yes, please register directly with the lecturers for the courses and exams and ask them to either send Mrs. Kaboub (kaboub@wiwi.hu-berlin.de) your grades or issue you a transcript (after you took the exam) that you forward to her, as all the grades are booked in  AGNES at HU. To enroll at the other universities, see the questions under Multiple Enrollment.